Utilities that operate nuclear power plants have made strides in operating efficiency and safety. But their red-hot shares could begin to malfunction.

NUCLEAR POWER IS lighting up Wall Street these days, attracting investors who once kept a safe distance from utilities that focus on this controversial energy source.

Twenty-five years after the Three Mile Island accident placed a cloud of suspicion over the U.S. nuclear power industry, many plants are enjoying a dramatic boost in operating productivity along with an improved reputation for safety. And these gains are helping the...